The individual and also serial machining of workplaces in modern cutting machine tools calls for maximum accuracies and the strict observance of closest tolerance ranges. In order to achieve and observe these high specifications, the components involved in the machining process, that is, the workpieces including their workpiece carriers (pallets) and also the tools including their tool holders and the work spindles, must be positioned and locked with high precision. The chips which are produced during the machining operation by the tool and which fly around in the work area and deposit on the components are of special significance. Particular problems will result when chips and other contaminations stick to the usually finished surface pairs of tool cone and tool holder or the front-side plane surfaces, or both, and these accumulated chips are pressed in firmly when the tool is exchanged the next time. Since the clamping forces applied to the tool shafts by the clamping devices of the work spindle can reach extremely high values, chips clamped between the inner wall of the tool holder and the outer wall of the tool shaft cause minor deformations of the spindle head, which can be the reason why the tool runs out of true and can be the cause of other faulty machining operations.
In addition, similar problems may occur when the finished plane surfaces have minor defects or also chip deposits on the front side of the tool holder, which can be made as full areas or also subareas. When the tool shaft is pulled into the spindle, the accumulated chips or other deposits are located between the two superfinished plane surfaces. As a result, the tool axis is tilted or offset with respect to the spindle central axis by small amounts, thus causing the tool to run out of true during the machining operation. The resulting defects on the machined tool can often only be determined by complex measurements of the entire tool, which requires a considerable technical effort and also involves the striking risk of producing considerable amounts of rejects.
Systems are already used by means of which the occurrence of imbalance phenomena can be detected on the tool as such, or also on the work spindle, to then take measures for avoiding or balancing, or both, the imbalance phenomena. They can consist in another tool replacement, for example, wherein the then exposed tool shaft and also the tool holder in the spindle can be cleaned by hand or also by a blowing-out operation. However, in order to carry out these operations, the work spindle must be accelerated and optionally be decelerated again only for the purpose of monitoring, thus requiring additional time and work.
The deformations of the tool holder, which are caused by compressed chips, and the resulting wobble movements of the tool holder, or also the spindle head, can be detected by optical systems as well, for example, by means of laser beams, the measurement data of which are prepared in electro-optical evaluating elements and supplied to the machine control.
EP 1 889 685 A discloses a machining unit for program-controlled cutting machine tools, the machining unit including a spindle housing which can be moved along the machine column and has a work spindle rotatably mounted therein. A collet which is axially movable in central fashion in the work spindle interior serves for clamping and releasing a supplied tool shaft in a tool holder on the front end of the work spindle. A monitoring device serves for controlling the spindle head shape and contains at least one measuring unit which already responds to minimal positional deviations that are caused, for example, by the deposit of chips on the respectively finished surface pairing of tool shaft and tool holder. The measuring device can contain mechanized, electric, electromechanical, acoustic, or optically acting sensors which detect the shape of the spindle head with always high measurement accuracy and the output data of which is processed in the evaluating unit. In this way, it is, for example, possible to detect a faulty positioning caused by deposited chips directly after the respective clamping operation of the tool without the work spindle having to be driven. The evaluating unit initiates via the machine control another exchange operation of the tool and a cleaning of the clamping surfaces, for example, by a blowing-out step. The measuring device used in this machining unit only detects deformations of the spindle head in the circumferential direction since the employed sensors detect the cylindrical lateral surface of the cylinder head and the shape deviations thereof.
Furthermore, DE 103 51 347 A discloses a measuring and monitoring apparatus for machine tools, which in a spindle housing movable along the machine column includes a work spindle rotatably mounted therein. Clamping elements in the work spindle interior serve for clamping and releasing an exchangeably fitted tool shaft in a tool holder. In order to ensure an accurate planar abutment of the tool shaft against the tool holder, an annular flange is mounted on the spindle head and has a plurality of equidistantly arranged finished subareas on the front face thereof. A sensor is attributed to each of these subareas and is connected to transmission elements for the measurement data transmission via inner connecting lines. The sensors and measuring elements only detect axial deformations which occur on the annular mating surface of the spindle head in the case of a non-flat abutment of the contact surface formed on the tool shaft. The energy and data lines extend in the respective components, that is, they are integrated therein. A limitation of this known measuring and monitoring system is that the sensor system and also the evaluating unit are made as a modular unit with the work spindle and can only be installed and removed together with the work spindle. This means in practice that a major maintenance and repair effort is necessary if malfunctions occur in the sensor system, that is, if one or also several sensors fail or the connections thereof are interrupted, or both.